This is a tough one.
It seems easy, you are not sending data or anything. The problem arises when you have to design a system that will only respond when it's suppose too.
I'll give you an example.
I work with underwater robotics, and I work with underwater acoustics. One of the underwater navigation systems works by single pings from acoustic transponder in known locations. Measure time of flight, triangulate, and you have your position. And these work at human audible frequency, and have ranges of kilometers.
One issues the system ran into early on was interference from snapping shrimp. The receiver could not make out if it was a ping from a transponder over the noise of snapping shrimp. The fix was to encode the pulse, so it stood out as a verifiable pattern over the noise.
So if you make a system for your pool, you will have to design it such that there is some method of verifying the source of the acoustic energy, so your 'light' doesn't come on when someone jumps in the pool.